Disorders affecting the cardiovascular system are major causes of morbidity and mortality in all developed countries. Treating these diseases accounts for a significant portion of health care resources. Further, temporary and permanent disabilities secondary to these diseases have substantial effects on the economic productivity of developed countries.
Accurate diagnosis and efficacious treatment of diseases affecting the cardiovascular system depend in large part on determining several parameters of cardiac function and on determining the blood flow to various tissues. The global blood flow is cardiac output. One useful way of determining regional or global blood flow is by measuring electrical bioimpedance in a body segment, including superficial organs. In the thorax, cardiac output is measured.
The electrical bioimpedance of a body segment depends upon a number of factors including the volume of blood or volume of fluid in the segment and the changes of electrical conductivity of the body segment. Several parameters of cardiac function and the hemoperfusion of the various tissues can be determined by measuring the magnitude of components of bioimpedance which are unchanging, and the rate and amplitude of changes in components of bioimpedance caused by arterial distension and blood flow.
Existing bioimpedance measuring systems utilize a continuous current generator to generate a continuous, constant magnitude measurement current through a human or animal body segment. Impedance to the continuous current flow in the body segment generates a voltage difference across the body segment. The amplitude of the voltage is modulated by changes in body segment electrical conductivity caused by changes in blood volume and velocity in the body segment which becomes an impedance transducer. The voltage across the transducer is measured by a bioimpedance detector, and the detector generates an output signal indicative of the impedance in the body segment.
The continuous, constant magnitude, measurement current used in existing systems has a magnitude ranging between 1 mA and 4 mA Root Mean Square (RMS). The signal frequency of the current used is between 20 kHz (one cycle period of 50 .mu.sec) to 100 kHz (period of 10 .mu.sec).
While functional, existing bioimpedance measuring systems are associated with some disadvantages. First, as stated above, existing systems utilize a continuous, constant magnitude, measurement current. This current is in a high frequency range and is delivered to living tissue for a protracted time. Though not definitively proven, high frequency current may be detrimental to living tissues including tissues present in the thorax.
Further, although the measurement current flows through the body segment continuously, the bioimpedance detectors utilized in existing systems sample the measurement current only 200 to 400 times a second (i.e. every 5 to 2.5 msec). Therefore, detrimental high frequency currents unnecessarily flow through the body segment between sampling.
A second disadvantage of existing systems is that the bioimpedance detectors utilized in such systems require several consecutive levels of amplifier circuits. Each amplifier circuit undesirably amplifies the input noise from signals detected in a body segment, thereby necessitating an increase in the magnitude of the measurement current to maintain an suitable signal to noise ratio. Multiple amplifier circuits require substantial area on printed circuit boards and utilize numerous circuit components thereby increasing the cost and power consumption of the system. The complexity of multiple amplifier systems decreases the reliability of the systems and increases the frequency of required maintenance.
A further disadvantage of existing systems is that the continuous, constant magnitude, measurement current they use can interfere with the operation of rate-responsive devices such as pacemakers.
Generally, the magnitude of the measurement current needed in bioimpedance measuring systems is determined by the level of noise in electronic circuitry in the bioimpedance detector. Reducing the magnitude of the measurement current also reduces the signal to noise ratio thereby decreasing the sensitivity of the system.
Therefore, it would be advantageous to have an electrical bioimpedance detection system that reduces exposure of a body segment to the measurement current without reducing the magnitude of the measurement current. Further, it would be advantageous to have an electrical bioimpedance detection system which does not expose the body segment to a continuous, high frequency measurement current. Still further, it would be advantageous to have an electrical bioimpedance detection system which does not interfere with rate-responsive devices such as pacemakers. Also, it would be advantageous to have an electrical bioimpedance detection system which would eliminate the need for multiple amplification circuits.